A brushless dc motor typically has a number of permanent magnets mounted on a rotor and a set of electromagnetic coils mounted on a stator. The rotor is made to rotate by energizing the coils in a specific sequence relative to the angular position of the rotor. The function of energizing the coils at specific points in the rotation is called "commutation" of the motor.
One typical means of commutation utilizes sensing devices (for example Hall cells) mounted on the stator for sensing the angular position of the rotor. Electronic circuits connected to the sensing devices detect the passage of the rotor past the sensing devices and switch the energy into a different electromagnetic coil (commutates) at that instant. Such prior art commutation means is well known and since it is relatively simple to implement, it is widely employed. However, such prior art commutation devices require the rotor sensing devices and, further, the efficiency of the motors utilizing such commutation devices depends critically on the precise placement of the sensing devices when the motor is manufactured. That is, if the sensing devices are not accurately positioned, the motor will not operate efficiently.
Another commutation means which is becoming more widely used is based on back electromotive force (EMF) sensing. When the rotor rotates, an electromagnetic field is induced into the coils which are not currently energized (the back EMF field). By sensing the back EMF field, the commutation points of the motor may be determined. Back EMF commutation has the advantage that it does not require the rotor sensing devices. However, back EMF commutation has the disadvantage of performance degradation when the motor is moving slowly or is idle. By nature, the strength of the induced back EMF is proportional to the speed of the magnets passing by the coils. At slow speeds or at idle, then, the back EMF field is weak and not easily sensed and the commutation may not be properly performed.
Many brushless dc motors are used in applications wherein motor speed is controlled by servo-control systems. Such servo-control systems require a feedback signal from the motor indicating the motor speed. Such feedback is typically supplied utilizing a magneto-resistive (MR) encoder. Such an MR encoder has a magnetized ring around the circumference of the motor rotor and an MR sensor mounted on the motor stator. As the motor rotor rotates, the magnetic poles in the ring pass by the MR sensor, generating a series of pulses having a frequency proportional to the angular speed of the rotor. MR encoders are typically magnetized to provide from fifty to several hundred pulses per revolution of the rotor.